Researchers Uncover Mechanism of Tobacco Carcinogen

NNK triggers epigenetic changes that result in an overexpression of DNMT1 and inhibition of tumor suppressors, as reported in JCI.

A group of investigators has discovered that nicotine-derived nitrosamine ketone (NNK), a tobacco carcinogen, not only causes DNA and genetic alteration but also induces epigenetic alterations in the process of cancer formation, inhibiting tumor suppressor genes.

The findings appear in a paper titled “The tobacco-specific carcinogen NNK induces DNA methyltransferase 1 accumulation and tumor suppressor gene hypermethylation in mice and lung cancer patients,” published in The Journal of Clinical Investigation. The research team was led by Yi-Ching Wang, Ph.D., department of pharmacology, National Cheng Kung University.

DNA methyltransferase 1 (DNMT1) catalyzes DNA methylation and is overexpressed in many human diseases including cancer. The tobacco-specific carcinogen NNK also induces DNA methylation. The role of DNMT1-mediated methylation in tobacco carcinogenesis, however, remains unclear.

To better understand this mechanism, the researchers used human and mouse lung cancer samples and cell lines to determine how NNK induced DNMT1 expression and activity and triggered the inhibition of tumor suppressor genes. They found that NNK weakened the degradation capacity of β-transducin repeat-containing protein (βTrCP) via the AKT pathway, enhancing the stability of DNMT1 protein. Moreover, NNK activated heterogeneous nuclear ribonuclear protein U (hnRNP-U), which allowed the translocation of the βTrCP proteins from the nucleus to the cytoplasm, inducing the accumulation of the DNMT1 proteins. The NNK-induced DNMT proteins then bound with the tumor suppressor genes, inducing the overexpression of DNAMT1.

In the animal model experiments, after the lung tissues of mice were resected and stained with H&E for microscopic evaluation, it was discovered that the βTrCP proteins mainly located in cytoplasm and the DNMT1 nuclear were misplaced.

The research team also used immunohistochemistry assays to detect the expression levels of DNMT1, βTrCP, and hnRNP-U proteins of the lung tissues in 124 cancer patients. The level of DNMT1 protein in patients who smoked continuously was higher than the level in patients who had already quit smoking.

The investigators thus suggest that βTrCP degradation pathway affects the expression level of DNMT1 protein, and NNK triggers the translocation of the βTrCP and DNMT1 protein, further enabling the excessive activation of the DNMT1 protein in the nucleus.